both your son and his maternal aunt's daughter were breastfed by the same woman mentioned above more than five times during their first two years of age, then it is forbidden for your son to marry her since she is considered his sister through breastfeeding ties. If they both or if one of them was breastfed by this woman less than five times during their first two years of age or after their first two years of age, then it is permissible for him to marry her. Allah (Exalted be He) says:
The mothers shall give suck to their children for two whole years, (that is) for those (parents) who desire to complete the term of suckling
Forbidden to you (for marriage) are: your mothers, your daughters
The Prophet (peace be upon him) said:
Milk relationships are equal to blood relationships in rendering marriage unlawful.
It was related that
`Aishah (may Allah be pleased with her) said: "It was revealed in the Qur'an that ten clear breastfeeding prohibit (marriage) then it was abrogated (and substituted) by five clear breastfeedings, and Allah's Messenger (peace be upon him) died while it was likewise." It is worth mentioning that if a baby (Part No. 21; Page No. 109) sucks milk from the breast - however little it may be - and then leaves it, this will be counted as one breastfeeding. If it again sucks milk from the breast - however little it may be - and then leaves it, this will be counted as a second breastfeeding, etc.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.